CN1277953C - Method for preparing strontium-barium titanate ferroelectric film - Google Patents

Abstract

The invention belongs to a preparation method of a ferroelectric thin film material. On the basis of the traditional Sol-Gel method, a seed layer is firstly provided on a substrate, and BST sol is repeatedly spin-coated on it to make a uniform BST thin film. The steps of the present invention: dissolve lead acetate Pb(C ₂ H ₃ O ₂ ) ₂ in a solvent composed of water and glacial acetic acid CH ₃ COOH, and dissolve tetrabutyl titanate Ti(OC ₄ H ₉ ) ₄ in ethylene glycol ether In C ₂ H ₅ O(CH ₂ ) ₂ OH solvent, the two parts are mixed and stirred to form PbTiO ₃ sol; PbTiO ₃ sol is added with N·N-formamide C ₃ H ₇ NO and ethylene glycol C ₂ H ₆ O ₂ as additives , the composition is sufficiently uniform to form a PbTiO ₃ seed layer precursor; spin-coat the PbTiO ₃ seed layer precursor on the substrate to form a wet film, irradiate with ultraviolet light to make it a solid film, and heat-treat to form an inorganic film; repeat the spin-coating steps 2 to 4 times, after heat treatment, the seed layer is formed; then the traditional Sol-Gel method is used to prepare a BST film on the seed layer. The BST ferroelectric thin film prepared by the invention has no cracks and holes, crystal grains grow obviously and distribute evenly, and can meet the requirements of the UFPA system for detecting materials.

Description

Method for preparing strontium barium titanate ferroelectric thin film

technical field

The invention belongs to a preparation method of a ferroelectric thin film material, in particular to the preparation of a high-performance barium strontium titanate BST ferroelectric thin film sensitive unit.

Background technique

Uncooled infrared detector focal plane array (UFPA) represents a new approach to the development of staring array thermal imaging, and is a key component in the manufacture of uncooled thermal imagers. Both can be completed on a common-scale integrated circuit process line, without having to rely on extremely expensive very large-scale integrated circuit (VLSI) manufacturing equipment. The key technology for preparing the focal plane array of uncooled infrared detectors is the preparation of high-performance detection sensitive element and its micro-insulation structure, and the high-performance ferroelectric thin film material is the premise to ensure the high detection rate and stability of the device. Its main function is to cause a change in the spontaneous excitation intensity of the material when the external temperature changes through the pyroelectric effect of the thin film material, and provide a corresponding detection signal to the detection circuit.

Barium strontium titanate (Ba _1-x Sr _{x )} TiO ₃ ferroelectric film, abbreviated as BST film, is a typical perovskite structure composite material, which combines the high dielectric constant, low dielectric loss and SrTiO ₃ has physical and chemical stability, semiconductor performance, and also has excellent ferroelectric, piezoelectric, pyroelectric properties and nonlinear optical effects. Therefore, in recent years, various devices prepared by using BST films have been widely used in the fields of ultra-large-scale dynamic random access memory, high-frequency bypass integrated capacitors, decoupling capacitors, microwave monolithic integrated circuits (MMICs), and tuned microwave devices. It shows obvious advantages in the preparation of focal plane arrays of high-performance uncooled infrared detectors. Methods for preparing BST thin films include radio frequency sputtering, organic chemical vapor deposition (MOCVD), pulsed laser deposition (PLD) and sol-gel (Sol-Gel) techniques. Compared with other methods, the film prepared by the Sol-Gel method has unique advantages: 1) Good uniformity. The raw materials reach the molecular level contact, especially for multi-component products, which can ensure the correct stoichiometric ratio and form a single crystal structure; 2) It is easy to carry out micro-doping to improve the performance of the film; 3) The heat treatment temperature is low, and it is easy to be compatible with the semiconductor integrated circuit process. The thin film and the semiconductor device can be integrated on the same chip to improve the reliability of the device and reduce the volume; 4) a large-area high-purity thin film can be prepared cheaply, and it is easy to industrialize. However, when the BST film is prepared on the substrate by the traditional Sol-Gel method, the crystallization of the BST film is difficult because the precursor solution of the BST film is not easy to nucleate and grow on the substrate. Experiments have found that the wetting angle between the colloid configured with pure organic components and the silicon wafer and quartz substrate is very large, and it is difficult to coat on the substrate (the sol is spin-coated on the substrate in the form of fine water droplets on the substrate). Although the sol can be coated on the substrate after the substrate is treated, the surface potential of the substrate surface is not uniform. , and can not form a uniform film, there are still some problems to be solved in order to obtain a practical BST film: 1) the crystallization state, the best crystallization state of the BST film is far from the crystallization state of the lead-based ferroelectric film; 2) BST thin films exhibit PMO-like relaxation ferroelectricity and diffuse phase transition characteristics, which make it difficult to determine the composition and T _max of BST thin films that are finally used as detection materials for uncooled infrared focal plane array UFPA systems.

Contents of the invention

The invention provides a method for preparing strontium barium titanate ferroelectric thin film, its purpose is, on the basis of the traditional Sol-Gel method, firstly provide a layer of smooth surface on the substrate, uniform surface potential everywhere, and The crystallization layer that is compatible with the sol is used as a seed layer, and then the BST sol is repeatedly spin-coated on it to make a uniform BST film.

A kind of method for preparing barium strontium titanate ferroelectric thin film of the present invention, its steps are as follows:

(1) The concentration of barium strontium titanate ferroelectric BST solution is Xmoll ^-1 , then the concentration of the seed layer sol is X±0.2moll ^-1 , where X is 0.3-0.5;

(2) When the concentration of the seed layer sol is 0.1～0.7moll ^-1 , for every 50ml of seed layer sol prepared, dissolve 0.005-0.035mol of lead acetate Pb(C ₂ H ₃ O ₂ ) ₂ in 1-3ml of water and 10- In a solvent composed of 16ml glacial acetic acid CH ₃ COOH, an equivalent amount of 0.005-0.035mol tetrabutyl titanate Ti(OC ₄ H ₉ ) ₄ was dissolved in 10-16ml ethylene glycol ethyl ether C ₂ H ₅ O(CH ₂ ) ₂ OH solvent, then mix the above two parts and heat while stirring to form PbTiO ₃ sol;

(3) Add the same amount of N·N-formamide C ₃ H ₇ NO and ethylene glycol C ₂ H ₆ O ₂ to the above PbTiO ₃ sol, each 4-8ml as an additive, heat while stirring, so that the components are fully uniform, Formation of PbTiO ₃ seed layer precursor;

(4) Spin-coat the PbTiO ₃ seed layer precursor on the Pt/Ti/SiO ₂ /Si substrate to form a wet film, irradiate it under an ultraviolet lamp to make it a solid film, and then heat treat it to form an inorganic film;

(5) Repeat step (4) 2 to 4 times, and then heat-treat to form a smooth and uniform surface with good adhesion properties;

(6) The traditional Sol-Gel method is used to prepare barium strontium titanate ferroelectric BST film on the seed layer, and the annealing of the BST film is carried out in the rapid thermal treatment furnace RTA respectively.

The method for preparing barium strontium titanate ferroelectric thin film is further characterized in that: in the step of spin-coating the PbTiO _seed layer precursor on the Pt/Ti/SiO ₂ /Si substrate, the uniform glue speed is 7000-9000r/ min, homogenizing time 5-30s, irradiate the wet film under ultraviolet light for 3-15min to make it into a solid film, heat treatment at 250-350℃ for 10-20min to form an inorganic film.

In the method for preparing barium strontium titanate ferroelectric thin film, the heat treatment temperature can be 500-700° C. and the treatment time can be 10-30 minutes after the number of repetitions of spin coating the PbTiO ₃ seed layer precursor to form an inorganic film is completed.

On the basis of the traditional Sol-Gel method, the invention adopts a two-step film-forming process of adding a crystal seed layer to prepare the BST thin film. The BST film was directly prepared on the substrate by the traditional Sol-Gel method, and its structure is shown in Figure 1(a). Since the precursor solution of the BST film is not easy to nucleate and grow on the substrate, it is difficult to crystallize the BST film. As shown in Figure 1(b), when a layer of PbTiO ₃ seed layer is made on the substrate, since PbTiO ₃ has c-axis crystallographic orientation, the preparation of BST ferroelectric thin films on this basis is easy to be induced by crystallization. A highly oriented BST ferroelectric film is formed.

The BST ferroelectric thin film prepared by the "two-step method" has no cracks and holes, and the crystal grains grow obviously, and the grain distribution is uniform, which can meet the requirements of the detection material of the UFPA system.

Description of drawings

Figure 1(a) is a schematic diagram of the BST film structure directly prepared on the substrate by the traditional Sol-Gel method.

Fig. 1(b) is a schematic diagram of the structure of the BST thin film prepared by the present invention.

Fig. 2 is a process flow chart of the present invention for preparing a BST thin film seed layer.

Fig. 3 is a flow chart of the process for preparing BST thin films in the present invention.

Figure 4(a) is the SEM image of the BST thin film prepared under the condition that the seed layer precursor concentration is 0.40moll ^-1 .

Figure 4(b) is the SEM image of the BST thin film prepared under the condition that the seed layer precursor concentration is 0.20moll ^-1 .

Figure 4(c) is the SEM image of the BST film prepared under the condition that the seed layer precursor concentration is 0.60moll ^-1 .

Detailed ways

The invention adopts a two-step method, introduces a seed layer and prepares BST thin films with different concentrations of the precursors of the seed layer. Results of Examples The scanning electron microscope analysis of the BST ferroelectric thin film was carried out using a Japanese SX-40 scanning electron microscope (SEM). Figure 4(a), (b) and (c) are the SEM images of BST crystalline thin films prepared under the conditions of seed layer precursor concentration of 0.40moll ^-1 , 0.20moll ^-1 , and 0.60moll ^-1 , respectively. It can be seen from the figure that the introduced seed layer can promote the nucleation and growth of BST components, and the crystallization status of the film introduced with the seed layer has been greatly improved: the film has no cracks and holes, and the grains grow obviously. Among them, the seed layer prepared by the formula of Example 1 has the best effect on the crystallization of BST thin film, and the prepared BST thin film can be used as the preferred material of the uncooled infrared focal plane array (UFPA) sensor.

Embodiment one:

The amount of each component used to prepare a seed layer precursor solution of 0.40moll ^-1 (0.02mol solute/50ml solution) is as follows: 0.02mol lead acetate Pb(C ₂ H ₃ O ₂ ) ₂ , 0.02mol tetrabutyl titanate Ti(OC ₄ H ₉ ) ₄ , 15ml glacial acetic acid CH ₃ COOH, 14ml ethylene glycol ether C ₂ H ₅ O(CH ₂ ) ₂ OH, 1-2ml water, PbTiO ₃ sol prepared with the above formula, add N N-formamide C ₃ H ₇ NO and ethylene glycol C ₂ H ₆ O ₂ each 6-7ml are additives. When making the seed layer, the mixing speed is 8500r/min, the mixing time is 25s, and the wet film is irradiated with ultraviolet light for 7 minutes to make it a solid film, and then heat-treated at 270°C for 12 minutes to form an inorganic film, repeating twice Afterwards, heat treatment at 600° C. for 20 minutes, and prepare the seed layer according to the process shown in FIG. 2 .

The concentration of the BST precursor solution is 0.40moll ^-1 . Taking the preparation of 1mol BST20 (x=0.2) as an example, 0.80mol Ba(CH ₃ COO) ₂ (barium acetate), 0.20mol Sr(CH ₃ COO) ₂ ·1/2H ₂ O (strontium acetate), 1mol Ti(OC ₄ H ₉ ) ₄ (tetrabutyl titanate) as raw materials, 700-750ml ethylene glycol monoethyl ether (HOCH ₂ CH ₂ OC ₂ H ₅ ) as solvent, 750-800ml Glacial acetic acid (CH ₃ COOH) is used as a catalyst, formamide (HCONH ₂ ) and ethylene glycol (C ₂ H ₆ O ₂ ) each take 250-300ml as an additive, and then make seeds with the process flow shown in Figure 3 Layers of BST thin films were prepared on the substrates. Results of the Examples The BST ferroelectric thin film was analyzed by a scanning electron microscope (SEM) of the Japanese SX-40 type, and the SEM image of the BST crystalline thin film was obtained as shown in FIG. 4( a ). It can be seen from the figure that the BST film prepared on the seed layer with a concentration of 0.40moll ^-1 has obvious grain growth and uniform distribution, with a size range of 40-90nm and an average particle size of about 70nm.

Embodiment two:

The amount of each component used to prepare a seed layer precursor solution with a concentration of 0.20moll ^-1 is as follows: 0.01mol lead acetate Pb(C ₂ H ₃ O ₂ ) ₂ , 0.01mol tetrabutyl titanate Ti(OC ₄ H ₉ ) ₄ , 16ml Glacial acetic acid CH ₃ COOH, 16ml ethylene glycol ether C ₂ H ₅ O(CH ₂ ) ₂ OH, 1-2ml water, add N·N-formamide C ₃ H ₇ NO and ethylene glycol C ₂ H ₆ O ₂ Each 6-7ml etc. are additives. When making the seed layer, the mixing speed is 9000r/min, the mixing time is 15s, and the wet film is irradiated with ultraviolet light for 10 minutes to make it a solid film, and then heat-treated at 300°C for 10 minutes to form an inorganic film, repeating 3 times Finally, heat treatment at 550° C. for 25 minutes, and other conditions are the same as those in the examples to obtain the SEM image of the BST crystalline film as shown in Figure 4 (b). As can be seen from the figure, the crystallization of the film has been greatly improved. However, the grain growth is less obvious.

Embodiment three:

The amount of each component used to prepare a seed layer precursor solution with a concentration of 0.60moll ^-1 is as follows: 0.03mol lead acetate Pb(C ₂ H ₃ O ₂ ) ₂ , 0.03mol tetrabutyl titanate Ti(OC ₄ H ₉ ) ₄ , 13ml Glacial acetic acid CH ₃ COOH, 13ml ethylene glycol ether C ₂ H ₅ O(CH ₂ ) ₂ OH, 1-2ml water, PbTiO ₃ sol prepared by the above formula, add N·N-formamide C ₃ H ₇ NO and Ethylene glycol C ₂ H ₆ O ₂ each 6-7ml etc. are additives. When making the seed layer, the mixing speed is 8000r/min, the mixing time is 30s, and the wet film is irradiated with ultraviolet light for 12 minutes to make it a solid film, and then heat-treated at 320°C for 10-20 minutes to form an inorganic film, repeat After 2 times, heat treatment at 630°C for 25 minutes, other conditions are the same as those in the examples, and the SEM image of the BST crystalline film as shown in Figure 4(c) is obtained. It can be seen from the figure that although the crystal grains are large, the distribution uneven.

Claims (3)

1. A method for preparing strontium barium titanate ferroelectric thin film, its steps are as follows: (1) If the concentration of barium strontium titanate ferroelectric BST solution is Xmoll ^-1 , the concentration of the seed layer sol is X±0.2moll ^-l , where X is 0.3-0.5; (2) When the concentration of the seed layer sol is 0.1～0.7moll ^-1 , for every 50ml of seed layer sol prepared, dissolve 0.005-0.035mol of lead acetate Pb(C ₂ H ₃ O _{2 )} in 1-3ml of water and In a solvent composed of 10-16ml glacial acetic acid CH ₃ COOH, an equivalent amount of 0.005-0.035mol tetrabutyl titanate Ti(OC ₄ H ₉ ) ₄ was dissolved in 10-16ml ethylene glycol ether C ₂ H ₅ O(CH ₂ ) In ₂ OH solvent, mix the above two parts and heat while stirring to form PbTiO ₃ sol; (3) Add the same amount of N·N-formamide C ₃ H ₇ NO and ethylene glycol C ₂ H ₆ O ₂ to the above PbTiO ₃ sol, each 4-8ml as an additive, heat while stirring, so that the components are fully uniform, Formation of PbTiO ₃ seed layer precursor; (4) Spin-coat the PbTiO ₃ seed layer precursor on the Pt/Ti/SiO ₂ /Si substrate to form a wet film, irradiate it under an ultraviolet lamp to make it a solid film, and then heat treat it to form an inorganic film; (5) Repeat step (4) 2 to 4 times, and then heat-treat to form a smooth and uniform surface with good adhesion properties; (6) The traditional Sol-Gel method is used to prepare the BST thin film on the seed layer, and the annealing of the BST thin film is carried out in the rapid heat treatment furnace RTA respectively. 2. the method for preparing barium strontium titanate ferroelectric thin film as claimed in claim 1 is characterized in that: in step (4) Pt/Ti/SiO ₂ /Si substrate spin coating PbTiO ₃ seed layer precursor step , the uniform glue speed is 7000-9000r/min, the glue uniform time is 5-30s, the wet film is irradiated under the ultraviolet lamp for 3-15min to make it a solid film, and heat treated at 250-350℃ for 10-20min to form an inorganic film. 3. the method for preparing strontium barium titanate ferroelectric thin film as claimed in claim 1 or 2 is characterized in that: in step (5) spin coats PbTiO _Seed layer precursor forms after inorganic film repetition times finishes, heat treatment temperature 500-700℃; processing time 10-30min.

Images